As is heretofore publicly known, industrial vehicles and construction vehicles such as shovel loaders or the like are arranged so as to scoop earth and sand with a work implement such as a bucket mounted swingably through a boom on the front part of the vehicle body, actuate a tilting cylinder connected to the bucket so as to tilt the bucket towards the vehicle body, actuate a lifting cylinder connected between the boom and the vehicle body so as to lift up the bucket mounted on the leading end of the boom, and then transfer the earth and sand scooped by or taken in the bucket.
An example of prior art hydraulic circuit for use in operating a work implement and its function are shown in FIGS. 1A, 1B and 1C. In FIG. 1A showing prior art hydraulic circuit for operating a work implement, the fluid under pressure supplied by a hydraulic pump e is allowed through the action of a tilting control valve f to flow into a tilting cylinder d so as to drive the piston therein, and when the tilting control valve f assumes its neutral position, the fluid under pressure is allowed through the action of a lifting control valve g to flow into lifting cylinders b. (This is referred to hereinbelow as a tilting preferential circuit).
In this drawing, reference character f.sub.1 denotes a tilted position for the tilting cylinder d, f.sub.2 a neutral position therefor, and f.sub.3 a dumping position therefor. Whilst, reference character g.sub.1 denotes a raised position for the lifting cylinder b, g.sub.2 a neutral position therefor, and g.sub.3 a lowered position therefor. Reference character h denotes a boom kicking-out electrical detent for electrically actuating a boom kicking out device (not shown) adapted to automatically stop the upward movement of a bucket c (reference FIG. 1C) when the bucket is lifted up to a predetermined position.
The lifting control valve g and the tilting control valve f are adapted to be actuated by output pressures delivered by pilot valves i.sub.2, i.sub.3, i.sub.3 and i.sub.4, respectively. (i.sub.2, i.sub.3 and i.sub.4 are not shown) The pilot valves i.sub.1 and i.sub.2 are connected through pilot circuits j.sub.1 and j.sub.2, respectively, to both ends of the lifting control valve g, whilst the pilot valves i.sub.3 and i.sub.4 are connected through pilot circuits j.sub.3 and j.sub.4, respectively, to both ends of the tilting control valve f. Reference character O denotes a pressure control valve for the pilot valve i.sub.1, and P a pilot pump for the latter.
FIG. 1B shows an embodiment of the relationship between the manipulation of a work implement operating lever and the bucket load when earth and sand scooping operation is made by a vehicle having a tilting preferential type hydraulic circuit for operating the work implement. In this drawing, "lifting" in the periods of time of I and III means lifting of a lifting arm a (Refer to FIG. 1C), "tilting" in the periods of time of II, IV and VI means tilting of a bucket C (Refer to FIG. 1C) to the side of the vehicle body, and "dumping" in the period of time of V implies the turning of the bucket reverse to the "tilting".
As can be seen from the drawing, "lifting" and "tilting" of the bucket are repeatedly made to scoop earth and sand thereby in such a manner that the bucket loading does not exceed the maximum fluid pressure, and in case the bucket is not fully filled with earth and sand in the course of scooping, the bucket is turned back to a dumping direction so as to allow the object scooped thereby to get into the bucket. In the period V dumping operation, there has been a problem that a reduction in the vertical load Fv on the bucket causes a slip of front wheel tires ("t" in FIG. 1C).
Further, FIG. 1C is an explanatory view of a locus defined by the edge of the bucket in case the scooping operation described above with reference to FIG. 1B is made. In FIG. 1C, the curve indicated with reference character W shows the surface of earth and sand to be scooped by the bucket, the curve indicated with reference character A shows an ideal locus defined by the edge of the bucket, and the curve indicated with reference character B shows a locus defined by the edge of the bucket when the scooping operation described above with reference to FIG. 1B (using the prior art hydraulic circuit described hereinbefore with reference to FIG. 1A) is made.
To carry out this scooping operation, the operator manipulates alternately a lifting operation lever and a tilting operation lever (both of them not shown), or alternatively, in vehicles provided with a boom kicking out device (not shown) serving as a lifting position holding device, the operator used to perform the scooping operation by operating only the tilting operation lever while the bucket is held at its lifted position.
Out of the above-mentioned two methods of operation, the former operation method is merely troublesome repetition of the lifting and tilting operations, whilst the latter operation method has posed a significant problem that the holding position in the lifting control valve g is the maximum lifting position, and in the prior art hydraulic circuit for operating the work implement, the lifting speed of the boom when the tilting operation lever is released (in the period of time of IV in FIG. 1C) is so high that the moving speed of the bucket in the forward and upward directions cannot be controlled and sufficient amount of earth and sand cannot be scooped by the bucket, thus necessitating a useless operation such as the dumping operation to be made in the period of time of V in FIG. 1C.